Methods and apparatus for in-vivo and in-vitro measurements of blood glucose levels are known in the art. Generally, the methods and apparatus are relatively complicated and measurements of a person's blood glucose levels are usually performed in a clinic or laboratory with the aid of a technician and costs of the measurements are relatively high.
Methods and apparatus for determining blood glucose levels for use in the home, for example by a diabetic who must monitor blood glucose levels frequently, are available. These methods and associated devices are generally invasive and usually involve taking blood samples by finger pricking. Finger pricking is perceived as inconvenient and unpleasant and to avoid finger pricking diabetics tend to monitor their glucose levels less frequently than is advisable. Moreover, many conventional glucometers require routine purchasing of sample sticks and pricking needles, which is bothersome and adds cost to the user. There is a need for glucometers that are easy to use and that perform non-invasive in-vivo assays of blood sugar.
PCT Publication WO 98/38904, the disclosure of which is incorporated herein by reference, describes a “non-invasive, in-vivo glucometer” that uses a photoacoustic effect in which light energy is converted to acoustic energy to measure a person's blood glucose. Pulses of light at a wavelength for which light is absorbed by glucose is directed by the glucometer to illuminate a part of the person's body, such as a fingertip, comprising soft tissue. The light pulses are typically focused to a relatively small focal region inside the body part and light from the light pulses is absorbed by glucose and converted to kinetic energy in a region of tissue in the neighborhood of the focal region. The kinetic energy causes temperature and pressure of the absorbing tissue region to increase and generates acoustic waves, hereinafter referred to as “photoacoustic waves”, that radiate out from the absorbing tissue region. An acoustic sensor comprised in the glucometer contacts the body part and senses the photoacoustic waves. Intensity of the waves is a function of the concentration of glucose in the absorbing tissue region and their intensity as measured by the sensor is used to assay the glucose.
However, light is scattered by body tissue and even though the light is focused to a small focal region inside the body, the location and size of the absorbing tissue region are not accurately known. As a result, the generated photoacoustic effect and measurements of the person's glucose levels are not necessarily the result only of glucose concentration in the person's blood. Characteristics of the absorbing tissue region, such as density of blood vessels therein, that can affect concentration of glucose in the absorbing region are often not accurately known. Measurements of blood glucose levels can therefore be affected by unknown variables that substantially compromise the reliability of the measurements.
U.S. Pat. No. 5,941,821 describes another non-invasive in-vivo glucometer that uses a photoacoustic effect to assay blood glucose. Light at a wavelength at which glucose absorbs light is modulated at a suitable frequency and directed by the glucometer to illuminate a region of a person's body. Glucose in blood and interstitial fluid in tissue near the surface of the region absorbs the light and converts the absorbed energy to kinetic energy that heats the tissue. Temperature of the tissue increases and decreases cyclically in cadence with the modulation of the light. The alternate heating and cooling of the tissue results in periodic heating of air in contact with the surface of the illuminated region, which generates sound waves in the air. A microphone comprised in the glucometer provides measurements of intensity of the sound waves that are used to determine a concentration of glucose.